The problem of mechanization and automation of welding processes when laying main pipe lines is relatively old, however, up to now girth joints in pipe lines are welded by hand. This is due to the imperfection of conventional techniques and equipment as well as to the peculiar features of assembly of pipelines, consisting in welding each subsequent pipe or a plurality of previously welded pipes to the butt of a stationary main pipeline already assembled and indicates that this problem has not been solved as efficiently as is desirable.
When welding two or more short pipes, use is commonly made of stationary apparatus having immovable welding heads and a mechanism adapted to rotate the pipes being welded. The welding techniques and equipment are sufficiently adequate and substantially comply with the requirements placed thereupon. When such apparatus are used, the pipes being welded are synchronously rotated at a predetermined speed and thereby the joint sections to be welded are brought to the welding heads. However, such techniques and equipment are inapplicable in butt welding of the ends of main pipelines, particularly because the length of the main pipe line already assembled cannot be rotated.
When laying main pipelines, the joints are welded by moving one or more welding heads around the immovable pipe. Various apparatus are used for practicing this technique (cf. U.S. Pat. No. 3,777,103 and Japanese Pat. No. 49-2667). The above apparatus comprise a yoke embracing the pipeline and carrying welding heads rigidly secured thereto and provided with wire feeders. The yoke is connected with a means for rotation thereof around the pipeline being welded.
Such an arrangement provides for simultaneous welding of a girth joint with the aid of several welding heads while rotating the yoke around the pipeline. However, it may be advantageously used only in butt welding of pipes having a diameter of not more than 1000 mm and is impracticable when pipes of a larger diameter are to be welded, since the mass of movable parts, such as yokes, wire feeders, etc. sharply increases, hence the power capacity of the drive is to be increased as well.
More preferable in this respect are a method and an apparatus for welding girth joints by independently moving welding heads along a stationary track structure, as disclosed in U.S. Pat. No. 3,461,264.
The welding heads of the above apparatus are mounted on carriages having drives adapted to impart a translatory motion thereto. Each of the welding heads is provided with a welding wire feeder adapted to feed a welding wire normally to the surface of the pipe. However, the track structure does not embrace the pipe being welded along the whole circumference thereof, and thus it is impossible to weld the girth joint without displacing said track structure.
The most efficient arrangement at present are a method and apparatus for welding girth joints by independently moving welding heads along a closed annular track structure as disclosed in U.S. Pat. No. 3,277,567 and Austrian Pat. No. 283,861.
This method provides for movement of each of said welding heads around the pipeline along a respective section of the girth joint, feeding a welding wire to each welding head, and forming a circumferential weld along the whole of the joint. This method is characterized in that the welding heads are moved from the top point of the pipe downwards and fed with the welding wire at a right angle to the surface of the pipe. Welding is carried out under the conditions permitting the weld to be loosely formed.
An apparatus for carrying out the above method comprises an annular track structure composed of two semi-annular portions the ends of which are coupled by connectors, such as cylindrical hinges, enabling said semi-annular portions to be spaced apart. The annular track structure mounts carriages connected with drives adapted to impart a translatory motion thereto. Each of the carriages has a welding head mounted thereon and is provided with a welding wire feeder adapted to feed the wire through a nozzle oriented at a right angle to the surface of the pipeline. The drives of the carriages provide for a downward translatory motion of the welding heads.
An unquestionable advantage of both the method and apparatus of the prior art is that it is possible to completely weld the girth joints without displacing the track structure. However, concurrently with said advantage they suffer from a number of serious disadvantages. In particular, when welding lower and lateral sections of the girth joint under conditions permitting the weld to be loosely formed the molten metal flows out from the welding bath, which causes inadequate filling of the joint groove, fails to provide the required quality of the weld, and makes it necessary to weld the joint in several passes. It is clear that the latter factor adversely affects the efficiency of the method as well as of the apparatus. The downward travel of the welding heads is also responsible for the outflow of the molten metal.
In addition, due to repeated passes and, consequently, a respective number of layers of metal in the girth joint the physical properties thereof will deteriorate due to interlayer welding defects which are practically inevitable in any multi-pass welding process.